Bleeder screws do not leak, wheel cylinders do
I hate to be the bearer of difficult news but I have found that the new replacement wheel cylinders have bleeders that have a 12mm hex on them. I have not explored them as to whether the threads are metric, and I would imagine they would not be metric because the internal threads should be running pipe thread.
If you have a bleeder that is leaking it is most likely NOT the bleeder screw itself. The bleeder uses a tapered seat and the bleeder is harder than the master clylinder (or air pack) it screws into. 9 times out of 10, it is a piece of debris that has migrated into the space where the taper makes the seal, allowing the brake fluid to seep by. A shot of high pressure air should dislodge any particles of rust that have accumulated there. The rust is from inside the wheel cylinder normally.
In that rare case where the taper is damaged, I would wonder how it got damaged? If it was over tightened it is likely the tapered seat is malformed as well. While changing a wheel cylinder is a PITA it sometimes is the only real way to know your brakes are going to work properly. The other reason for changing a wheel cylinder is that the reason they leak is due to rust inside the bore of the wheel cylinder causing the leak at the bleeder and the rust is a timebomb waiting to happen as far as a major brake failure. Once the bore of a wheel cylinder is pitted and is deteriorating, the piston can stick, leak by the seal or blow out all at once.
All of this says that when you are dealing with leaky bleeders you need to look to the cause of the leak, and it is definitely not the bleeder screw that caused the leak.
Just my two cents....
RL
